Modern drilling techniques used for oil and gas exploration employ an increasing number of sensors in downhole tools to determine downhole conditions and parameters such as pressure, spatial orientation, temperature, gamma ray count, etc., that are encountered during drilling. These sensors are usually employed in a process called “measurement while drilling” (or “MWD”). The data from such sensors are either transferred to a telemetry device and thence up-hole to the surface, or are recorded in a memory device by “logging”.
The oil and gas industry presently uses a wire (wireline), pressure pulses (mud pulse—MP) or electromagnetic (EM) signals to telemeter all or part of this information to the surface in an effort to achieve near real-time data. The present invention is specifically useful for a certain class of MP systems, although it can be useful in other telemetry or downhole control applications.
In MP telemetry applications there is a class of devices that communicate by a rotary valve mechanism that periodically produces encoded downhole pressure pulses on the order of 200 psi. These pulses are detected at the surface and are decoded in order to present the driller with MWD information. These rotary valves are preferentially driven by electric gearmotors.
The rotary valve mechanism comprises a stationary component and a rotating component. The stationary component, the “stator”, has fluid pathways for the drilling fluid as it is forced down the pipe housing the pulser. A second component, the “rotor”, is designed such that it can rotate to line up with the stator to create “open” and “closed” positions; when the rotor moves to the “closed” position the fluid pathway area is significantly restricted, causing the fluid velocity to increase in the vicinity of the rotor/stator assembly. This process is further described in U.S. Pat. No. 3,739,331.
The rotating component typically utilizes a shaft connected to a drive mechanism. This shaft is subject to abrasive conditions in the downhole environment due to the turbulent high velocity fluid flowing past; furthermore, this fluid is normally highly abrasive due to the inclusion of particulate matter such as sand. An example of a prior art MWD tool is shown in U.S. Pat. No. 3,982,224, where it can be seen that the drilling fluid can readily flow between the rotor and stator and erosion could result.
In summary:                the downhole rotary valve mechanism in most cases employs a rotary output shaft, and        the shaft is exposed to a highly abrasive environment causing erosion.        
What is required, therefore, is some means to protect the shaft associated with the rotor from erosion.
Conventional methods of protection have had only limited success. There have been some attempts to shield the shaft from erosion by creating a stepped edge from the stator that the rotor slides over (as is taught, for example, in U.S. Pat. No. 4,914,057) but this type of technique adds significant mechanical complexity and cost.